Cst purchase invoice of 31-3 but filed in next returns

In Andhra Pradesh My client is Dealing with VAT Purchase and Sales & CST Purchases not dealing with CST Sales. And my question is that he filed the VAT Returns in May for the month of April, where he included a CST Puchase Invoice dated 31-3-13, which is not mentioned in the March month returns. what shold i do to get the C Form for that Invoice